Measuring instrument



Sept. 29, 1942. p w cms-|- 2,297,346

MEASURING INSTRUMENT Filed Aug. 3, 1940 2 Sheets-Sheet l SePtQZQ, 1942. *l P. w. cls

M'EA'sURING INSTRUMENT Filed Aug. 3, 1940 2 sheets-sheet 2 Patv'cnted Sept. 29, 1942 MEASURING msTRUMEN'r Philip W. Crist, Pittsburgh, Pa., assixnor to Pittsburgh Plate Glass Company, Allegheny County,v Pa., a corporation of Pennsylvania Application August 3, 1940, Serial No. 350,946 3 Claims. (Cl. 175--183) The present invention relates to processes of and apparatus for determining dielectric value, thickness, state or condition and such like properties of a material, and it has particular relation to processes and apparatus designed for 4making such observations by measurement of the effect of the material upon the capacity of a set of juxtaposed electrodes arranged to provide an electrostatic condenser with the material in the field thereof.

One object of the invention is to provide a process of and apparatus for determining dielectric value or characteristics, which influences dielectric value, of a material that is in the form of extensive sheets or bodies or is so situated or conditioned that the values cannot be determined conveniently by conventional methods and apparatus.

A second object isito provide1 such apparatus in inexpensive, simple, compact and highly portable form.

A third object of the invention is to provide a process and apparatus of the foregoing type which is not subject to a disturbing influence of bodies in the neighborhood in which determinations'are being made, or by other factors likely to be present.

These and other objects of the invention will be apparent from consideration of the following specification and the appended claims.

The determination of dielectric value and other values of materials, notably in the form of extensive sheets or bodies so mounted upon supports as to be but partially accessible, often present serious difliculties, because in most determinations it is necessary to obtain access simultaneously to both surfaces of the material to be measured. One method proposed for making such determinations involves juxtaposition of spaced metallic conductors adjacent to opposite faces of the material in such manner that the conductive bodies constitute an electrical condenser whose capacity is a function of the dielectric constant and the thickness of the material disposed in the field thereof. Upon application of a high frequency alternating electrical voltage to the conductive bodies the variations in the capacity of the condenser are easily observed and from them the properties of the materials can be estimated. In this way such properties as dielectrical value, thicknesses of the material between the conductors and various other properties 'can be determined. Apparatus of this type, however, is bulky and usually is of is often subject to influences of bodies adjacent thereto and is subject to other variables which often are difficult to predict. In many instances, as for example, when it is desired to determine thickness, or dielectric value of a sheet of glass cemented upon a polishing car or table, this type of measurement is practically, if not completely, impossible, because of inability properly to juxtapose a suitable conductor to the surface which is cemented to the table or car.

Inv accordance with the provisions of the present invention it is proposed to obviate the foregoing diiiiculties b y provision of a condenser including a pair of laterally spaced conductors, or electrodes. which can be juxtaposed to a single'I surface of a sheet of material and which, when subject to an alternating voltage, induces an electrostatic field, which passes through the material from one side and again returns to emerge from the same side. By means of apparatus of this type a dielectric value and properties upon which such values depend can easily be determined even in bodies in which access only to a single surface can be obtained. Moreover, by suitable arrangement of the conductors of the condenser it is possible greatly to reduce or to minimize the eiects of neighboring bodies or the movement of' neighboring bodies upon the capacity of the condenser.

For a better understanding of the invention reference may now be had to the accompanying drawings in which Figures 1 and 6 are wiring diagrams of suitable circuits in which the condensers constituting the subject matter of the invention may be employed.

Figure 2 is a fragmentary cross-sectional view of one form of condenser or capacitance involving the principles of the invention.

Figure 3 is a plan view of the condenser shown in Figure 2.

Figure 4 illustrates the application of the invention to the measurement of properties related to dielectric value of a sheet of glass upon a polishing car.

Figure 5 is a fragmentary view showing a modified form of capacitance for use in practicing the invention.

In vthe drawings numerals refer to like parts throughout.

The apparatus 9, as shown diagrammatically in Figures 1, 2 and 3, involves a capacitance constructed in accordance with the present invention and connected into a circuit designed to amplify minute changes in current in the circuit,

necessity of stationary character. Moreover, it "5 due to changes in the dielectric value of the material to be tested and' then to determine the value of such changes. Dielectric value as herein employed depends upon the ratio of dielectric constant of the material to the thickness. The apparatus includes as a main element a capacitance IU constructed in accordance with the provisions of the invention and including a pair of conductor plates or electrodes II and I2, which are suitably and laterally spaced and insulated from each other, but are preferably disposed approximately in a common plane. The device, for purposes of convenience, may be termed a coplanar condenser because its plates are in a single plane. It may include a central disk constituting the electrode II and an annular disk I2 disposed thereabout. These disks are suitably secured in insulated relation with respect `to each other to a backing I4 of an insulative material, such as Bakelite or other non-conductive plastic. The lower face of one or both of the disks is provided with small projections I5 designed to rest upon the surface of a material, such.as a sheet of glass I6, which is to be tested and to maintain the spacing between the surface and the of free water, as well as more or less soluble salts,

which impart thereto electric conductivity. This is particularly true in grinding and polishing operations where the glass is subjected to re peated applications oi' a water slurry of sand, and a certain amount of water of necessity obtains contact with any exposed plaster, thus keeping the latter constantly wetted.

From Figure 2 it will be apparent that if anv alternating electrical voltage is impressed upon one` of the disks II or I2 an electric field will be generated in the vicinity thereof and a portion of this eld will travel downwardly through the dielectric material, e. g., a sheet of glass Il and will induce electrostatically a voltage in the conductive cement I1, which in turn will cause an electrostatic field to be generated, which will influence the other disk of the pair.

In order to induce an alternating voltage of high frequencyl upon one of the plates, e. g. the

" plate Il, and also to admit of determination of the variable capacity between the elements II and I2, the conductors are connected in circuit with a generator 22 of high frequency or radio frequency alternations, which also acts in effect as an amplifier to increase the sensitivity of the apparatus to small changes in the capacity of unit III, due to the effect oi the material to be tested. The circuit includes conductors 23 and 24 connected respectively to the conductors or disks II and I2 and arranged in series witha pair 'of condensers l24 and 21, preferably of variable type. These two condensers are also connected in series with each other by means of a conductor 28, which is grounded to a metallic case 30, providing 'a shield about the electrical n instrumentalities of the device. Since conductive material I'I, backing plate Ii, is grounded to the earth through the car I8, a Wheatstone bridge effect is obtained, in which the capacity between conductor Il and plate II and that between conductor 1 and plate I2 provide one side.

The branches ofthe bridge are joined by a conductor 2| containing a condenser Sla with the plate 32 of a conventional three electrode or triode electronic valve or radio tube I3, having a control grid 34 disposed between the plate and its filament Il. A positive plate voltage may be impressed on the plate 22 by means of a battery Il, which is connected to conductor 3| by means of conductor 3l, micro-ammeter 3l and a radio frequency'choke coil 4 I The battery is also connected to supply filament current to filament 8l by means of conductor 42 and the control rheostat 43 and choke coil 42a upon one side and by means of conductors 44, 4l, and 41 upon the other side'.

Control grid 24 is connected by conductor 48 to one electrode 4I of piezo crystal 5I of quartz or other suitable material. The opposite face of the crystal contacts with a second electrode B2, which is connected by conductor Il, with conductor 4I, to form a grid circuit. The crystal is also connected in parallel with aradio frequency choke coil 54 and a resistance il, by means of which the resonance characteristics of the grid circuit may be determined and controlled.

In order to facilitate maintenance of resonance between the grid circuit and the plate circuit of the electronic valve or tube, a radio frequency coil 51 is connected to conductors 23 and 24.

In the operation of the apparatus the condensers 28 and 21 are adjusted to obtain a Wheatstone bridge balance with the elements of the condenser or capacitance III. Buch balance when attained is permanent and need not be made again. The grid and plate circuits are also adjusted to obtain resonance when the capacitance of unit III is at some predetermined value. When the proper adjustment has been attained the condenser I0 may be disposed above a surface of any body which is to be tested, e. 8. the plate of glass I6. It VWillvbe manifestthat assuming that the plate circuit has been adjusted to obtain resonance with the grid circuit with the unit Il at its natural capacity, any change in that capacity, for example such as may be induced by placing the discs or plates II and I2 adjacent to plate I6, will tend to reduce the resonance of the circuit and produce avariation, for example, a reduction in the plate current as measured by the micro-ammeter 2l. The total dielectric value of the plate-like body Ii isa function of the thickness. By calibration of the instrument with plates of known thickness, it is possible to determine the thickness of a test plate merelyfrom the meter reading. Thickness or dielectric constant may also be approximately determined by the formula:

In this formula C equals the capacity of the condenser, K is the dielectric constant of the sheet,` A is the area of the electrodes or plates, X equals the thickness of the dielectric sheet, r of course lrepresents the ratio betweenthe circumference and the diameter of a, circle and may be assumed to be 3.14. Assuming that C is determined by means of the apparatus and K and A are known, X, the thickness of the plate canbe calculated.

In Figure 5,of the drawings is shown the modiiled form 59 of coplanar capacitance suitable for use in place of the unit Il. It includesl a pair of comb-dike elements III and 6I having spaced teeth i2,which intermesh, but do not physically contact with each other. Elements III and II are also provided upon their lower surfaces with protuberances 63 which function as do protuberances I to support the device in a definite spaced relation with respect to the surfaces to be tested. The relationship of the two elements 60 and I may be maintained in any convenient manner. for example, by means of a plate of non-conductive plastic superposed thereupon and secured thereto by means of screw fastenings 64. The capacitance is connected as indicated to the conductors 23 and 24, shown in the circuit in Figure 1.

In Figure 6 is shown the use of the principles of the invention in connection with an amplifying and measuring circuit designed for operation from conventional alternating current supply. In this embodiment of the apparatus the capacitance employed in generating an electrostatic eld in the material to be tested may be of the forms shown in either Figures 3 or 5, or of any other equivalent construction and is indicated in the drawings at 10. It includes coplanar conductors or electrodes 12 and 13, which in the actual physical embodiment of apparatus are approximately inthe same plane and are designedto rest upon or near to a single surface of a material. such as sheet I6 of glass, which is backed by a suitable conductive material, such as layer I1 of plaster of Paris, the latter of course is suitably grounded, -as indicated at 1l by the car upon which it is disposed. 'I'his condenser is connected by lines or conductors 1l and 11 in parallel circuit with a radio frequency coil 18, which is tapped at an intermediate point 19 by line 8|, so that the coil and coplanar condenser 1l provide a balanced bridge. Line II, also, constitutes a supply line for the alternating current employed to energize the circuit. Line l2 having a grid condenser and a grid leak 84 connects line I1 to the grid I8 of a conventional triode tube 81. 'I'he plate Il in the latter is connected by line 8l containing radio frequency choke coil 9| to milliammeter 92 in current supply line 8l.

I'he plate circuit is preferably tuned to resonance with the circuit of coil 1I and condenser unit 10 by means ofa coil N and condenser n. which are interconnected at their poles by conductors l1 and Il. The latter is Joined by conductor l! in circuit with condenser IBI to the plates of supply line u. The line 91 between coil Il and condenser 96 is connected by line |02 to one side, III of filament I 'I'he same side of the filament is also connected by resistance I and parallelly connected condenser "I1-and line III with line 1I of the bridge circuit. Ihe other end of the filament I is Joined by line or conductor III, choke coil II I and resistance H2 to current supply line I3.

In the operation of this embodiment of the apparatus it is to be imderstood that the bridge circuit containing the unit 1l is so adi usted that electrical balance is obtained between the ground 1l andthe connecting point 1l of the supply line II.- 'I'his balance is maintained permanently. 'Ihe tuned circuit containing the coil Il and condenserisalsosoadiustedastoobtainres- -onance between the plate and the grid of the electronic valve or triode tube I1. When these conditions are obtained the unit 1l may be suitably disposed with the conductors 12 and 1I adiacenttotbesurfaceof thematerialtobetested. e.g.aaheetofglass. Anyvariationof capacity inmeunitllduetotheintroductionofthedielectric material I6 and the conductive material I1 into contiguity with the conductors 12 and 13, of course increases or decreases the total capacity of the grid circuit of the tube, and therefore tends to throw it out of resonance with the plate circuit. The disturbance in resonance changes the flow of current in the plate circuit, which change is indicated by the milliammeter 92. Since the changes in capacity of the unit 10 are proportional to the thickness of the material I6,

it will be apparent that the current. recorded by oi' the valve I1.

the ammeter 92 varies with the thickness of the material tested.

By proper calibration of the instrument by samples of known thickness it is possible readily to determine the thickness of large sheets of material, such as sheets of glass, even when one surface is completely inaccessible. The readings may be made continuously or intermittently even while the cars are in motion in the polishing line.

'I'he system herein disclosed may be used for many purposes. such as determination of dielectric constant of dielectrical materials of known thickness, thickness of materials of known dielectric constant. The system may, also, be applied to study of paint or varnish lms on a metal surface. In such case the coplanar capacitance unit is applied adjacent to the surface of the nim while the metal backing is grounded or not,

as preferred, and a reading is made which by comparison with calibration curves can be interpreted. Films of rust, between the metal and the paint, air bubbles, in orl under the illms and fiaking of the film are manifested in the readings obtained from the system.

'I'he forms of the invention herein shown and described are to be considered merely as exemplary. Numerous modiiications may be made therein without departure from the spirit of the invention or the scope of the following claims.

What I claim is:

1. A method of determining dielectrical properties and properties upon which they depend in dielectrical material adhering to a conductive backing, which method comprises grounding the backing and making it a common electrode oi a pair of capacitances in bridge circuit, impressing a high frequency voltage on the circuit and determining the effect of the overall capacity of the bridge circuit upon the Supply of high frequency voltage.

2. A method of determining dielectrical and related properties in a dielectrical material adherent upon a conductive backing, which method comprises making the backing a common electrode of a pair of capacitances in a bridge circuit in the circuit of a tuned generator of high frequency voltage, and observing the effect of the capacity oi' the capacitances upon the current due to changes in the tuning of the circuit.

3. A method of ldetermining thickness of a glass plate while it is bonded by means of an intermediate layer of plaster to the top oi a grinding and polishing car, which method comprises iuxtaposing a pair of electrodes to the exposed surface of the plate in position to make of the layer of plaster a common grounded electrode of a pair of capacitances. said pair of electrodes being connected in a tuned circuit containing a source of high frequency voltage. the eil'ect of changes in tuning of the circuit due to the presence of the capacitances on the circuit.

PHILIP W. CRIBT. 

